We are studying murine intracisternal Type-A particles as a model for the expression of integrated reiterated viral genes. Using blot-hybridization techniques, we have identified a characteristic set of A-particle-related DNA fragments generated when mouse genomic DNA is digested with restriction enzymes and fractionated by electrophoresis in agarose gels. Several major restriction fragments have been cloned by insertion into plasmid pBR322 and transfection of E. coli chi1176. A-particle genes are not arranged in regular tandem arrays in the mouse DNA and appear to be heterogeneous with regard to both their internal structure and their flanking sequences. A structural relationship between intracisternal A-particles and an extracellular retrovirus (M432) of Mus cervicolor has been confirmed by immunological analysis of the particle proteins.